Commercial nuclear fission reactors for generating power normally comprise a core of fissionable fuel wherein the fuel material is sealed within tube-like metal containers. These tubular containers with the fuel are arranged or grouped in discrete bundles or units, which frequently are enclosed within an open ended housing known as a "channel" in the nuclear fuel industry. The discrete fuel bundles are assembled for service within the nuclear reactor to provide the core in predetermined patterns The assembled bundles are spaced apart from each other so as to provide intermediate gaps between each bundle, forming a surrounding area for the flow of coolant thereabout and also the insertion of reactor control means comprising neutron absorbing material.
Nuclear reactor control means typically consist of components containing neutron absorbing compositions which are reciprocally movable in relation to the core body of neutron emitting fuel undergoing fissions. The rate of the fission reaction, and in turn heat generated, is regulated by governing the availability of fission produced neutrons for furthering the fission reaction and determining the magnitude of the reaction.
In a conventional nuclear reactor, fissionable atoms such as uranium isotopes and plutonium absorb neutrons in their nuclei and undergo a nuclear disintegration or splitting. This fission produces on the average of two products of lower atomic weight and greater kinetic energy, and typically two or three neutrons, also of high energy.
The fission neutrons thus produced diffuse through the core containing fissionable fuel and they are either utilized or lost in several distinct competing mechanisms. Some neutrons may migrate to the boundaries of the core and escape whereby they are lost from the system. Some neutrons undergo nonfission or radiative capture in the fuel material. Other neutrons undergo fission capture within the fissionable fuel and thereby produce additional fission neutrons, the so-called chain reaction. Namely, fast neutrons are captured in the uranium 235 and 238, while thermal neutrons are captured in uranium 235. Still other neutrons undergo parasitic capture in the various extraneous or nonfissionable compositions of the core and adjoining components such as the moderator, coolant, various structural materials, fission products produced within the fuel, as well as the reactor control elements.
The balance between the fission production of neutrons and the various competing mechanisms for neutron consumption determine whether the fission reaction is self-sustaining, decreasing, or increasing. When the fission reaction is self-sustaining, the neutron multiplication factor equals 1.00, the neutron population remains constant, and on average there is one neutron remaining from each fission event which induces a subsequent fission of an atom.
Heat produced by the fission reactions is thereby continuous and is maintained as long as sufficient fissionable material is present in the fuel system to override the effects of fission products formed by the reaction, some of which have a high capacity for absorbing neutrons The heat produced by the fission reactions is removed by a coolant such as water, circulating through the core in contact with the tubular containers of fuel and conveyed on to means for its utilization, such as the generation of electrical power.
The neutron population, and in turn the heat or power produced, of a nuclear reaction, depends on the extent to which neutrons are consumed or wasted by capture in nonfissionable material. Neutron consumption of this nature is regulated by governing the relative amount of neutron absorbing control material imposed into the core of fissionable fuel undergoing fission reactions.
Control devices comprising elements containing neutron absorbing material, are commonly provided in the form of rods, sheets or blades. The elements are provided with mechanical or fluid operated means for reciprocal movement into and out from the core of fissionable fuel to any appropriate extent or depth for achieving the desired neutron population, and in turn, level of reaction
Common neutron absorbing materials include elemental or compound forms of boron, cadmium, gadolinium, europium, erbium, samarium, hafnium, dysprosium, silver and indium.
Commercial nuclear reactors for power generation are of such a magnitude that the control means, or systems, comprises a plurality of control units or rods. Each individual control unit or rod is selectively and reciprocally insertable to variable degrees of penetration into the fuel core by movement intermediate the discrete bundles of grouped tubular fuel containers through the spaces or gaps provided throughout the assembly of multiple fuel bundles. A common design for control rods, as shown in U.S. Letters Pat. No. 3,020,888, consists of an element having four blades, comprising sheaths containing neutron absorbing material, having a cross or cruciform cross section, whereby the four blades radially project at right angles to each other. With this design configuration, each control rod element is insertable into the spaces between four adjacent fuel bundles of the core assembly, and regulates the neutron flux or density emitted from the fissioning fuel of the four bundles.
The construction designs, materials, operating mechanisms and functions of typical control mean for water cooled and moderated nuclear fission reactors for commercial power generation are illustrated and described in detail in the prior art, for example, U.S. Letters Pat. No. 3,020,781; No. 3,020,888; No. 3,217,307; No. 3,395,781; No. 3,397,759; No. 4,285,769; No. 4,624,826; and No. 4,676,948, and elsewhere throughout the literature dealing with nuclear reactors. The contents of the foregoing prior art patents are incorporated herein by reference.